Process and device for using of low temperature heat for the production of electrical energy

ABSTRACT

The invention relates to the using of low temperature heat for the production of electrical energy by using of supercritical carbon dioxide as working fluid. 
     It included a process and a device for process realizing with higher efficiency in relation to other known processes and with a wide temperature working range. This is related to a wider adjustability which allows an optimal operation both in summer and in winter operation without technical or constructional changes. The process is realizable without damages to the environment and is realizable with low effort. The relative emission of carbon dioxide is reduced in relation to other processes. 
     Low temperature heat from a given heat source ( 1 ) is taken off by carbon dioxide at high supercritical pressure as heat transfer and working fluid in the process. Then the heated fluid is expanding in an expansion machine ( 2 ), which is connected with a generator ( 3 ) for the production of electric power. In this process the fluid will be cooled, then liquefied by using of a cold source ( 4 ) and in liquid state compressed to the working pressure.

The invention relates to the using of low temperature heat for the production of electrical energy by using of supercritical carbon dioxide as working fluid.

FIELD AND BACKGROUND OF THE INVENTION

There known two methods for the using of low temperature heat from burning and reaction processes as well as from solar thermal and geothermic processes essentially:

-   1. In the OCR (Organic-Rankine-Cycle)-process the heat is taking     away from the process medium by a heat exchanger and using for the     production of steam. Refrigerants, freezing or low boiling     substances as e.g. pentane are used in this process, vaporized and     labor-producing expanded due a expansion turbine connecting with a     generator. The expanded vapor is used for preheating and then     condensed. The heat of condensation will be delivered into the     surrounding. The efficiency is estimated by the difference between     the temperature of condensation (temperature of the surrounding) and     the reachable temperature of vaporization from 300 K to 625 K     depending of the used working fluid. The heat transfer is realized     by a silicon oil cycle normally. A changed version of the     OCR-process for low power is known as edc-process. The edc-process     is working with temperatures of condensation from nearly 248 K to     350 K and is using special constructed turbines. The efficiency of     the ORC-process is reaching at a temperatures level of 100° C.     nearly 6.5% and at a temperature level of 200° C. nearly 13-14%. -   2. In the Kalina-process the heat is taking away from the process     medium by a saturated ammonia-water mixture, in which ammonia is     desorbed. The ammonia is labor-producing expanded due an expansion     turbine which is connected with a generator. After them the cold     ammonia is reabsorbed in the ammonia-water mixture. The efficiency     of the Kalinin process is with 18% (brutto) a little higher than the     ORC-process, according to the literature. Profitable is the simpler     construction of the plant as well as the significant wider range of     temperatures of the working fluid. Disadvantageous are the problems     with the construction material by using of the aggressive     ammonia-water mixture which is leading to a lowering of the cycling     time of the little proved plant. Another disadvantage is the danger     of possible emissions of the high-toxic and environmentally     dangerous ammonia at possible leakages or disturbances.

Task Invention

Task invention is developing a process for using low temperature heat for the production of electric power with a higher efficiency and a broader working range as known processes and a simple device for the realization of this process with a low material effort and with a low environmental hazard.

The wider working range and wider adjustment range are loading to optimal processing in relation to local conditions and climate, e.g. summer or winter operation without constructive changes and minimization of the production of carbon dioxide.

According to the invention the task is solved by using high-pressured supercritical carbon dioxide as heat transfer medium, which takes off low temperature heat from a heat source, after them is expanded labor-producing in an expansion turbine, which is connected with a generator, as a result is cooled, then by using of a cold source is liquefied and in liquid state is pressured to the working pressure.

The process implied at least an external heat source (1), an expansion machine (2) connected with a generator (3), a heat exchanger with liquefier (4) and a pump (5) for compressing liquid carbon dioxide to supercritical pressures and a carbon dioxide storage (6) as soon as control devices and valves belonging to it, characterized in that carbon dioxide is used as the heat transfer medium and working fluid. The carbon dioxide will be liquefied at low temperatures, compressed in the liquid state to high supercritical pressures, at these pressures buffered for heat exchange process, taken off the thermal energy at this pressure from a heat source (1) and labor-producing expanded due an expansion machine (2) with an connected generator (3). In the expanding process carbon dioxide will be cooled and the final temperature will be controlled by the wanted pressure for the liquefaction. After them the carbon dioxide will be liquefied at this pressure by heat exchange a cold source due the discharge of the heat of condensation. The compression of the liquid carbon dioxide by a liquid pump (5) needs relatively few energy and a possible increase of temperature is acting in the increase of the efficiency.

In relation to the use of steam there are many advantages. First an expensive water purification plant will be not needed and then the relatively high losses in the waste heat boiler are avoided, which are resulted by the big temperature differences between the cooling-down curve of the exhaust gas and the warming-up curve of the steam through the vaporization of the water. The often used two-pressure and three-pressure cycles for a better adaption are causing a higher effort in material and control.

These difficulties will be avoided by the alternative use of the supercritical region for the heat transfer. The region is very interest too because their easy thermodynamic terms for the heat exchange by using of low temperature heat. That is caused by relatively high values of the heat capacity, low values of the viscosity and a heat conductivity comparable to steam.

The thermodynamic usable range of state is limited to low temperatures by the triple point of carbon dioxide at temperature of nearly 217 K and a pressure of 0.55 MPa. To higher temperatures and pressures there are no limitations for thermodynamic reasons, but for the practical use in relation to the material of the expansion machines and the heat exchangers.

An additional advantage to the OCR-process is given by the fact that the heat transfer and working fluid are not differently, carbon dioxide is used for both tasks. The fluid is working in a closed circuit and an additional heat exchanger is not needed.

Other advantages of this medium are given by its relatively low danger potential for people and environment and its high availability.

Besides them the storage of bigger amounts of carbon dioxide under control and its use as working fluid relieves the atmosphere and the environment and gives credits by the carbon dioxide trade. Other advantages are given by a higher efficiency of the process and in the possibility to combine the process with other heat and cold potentials increasing real efficiency. That is possible by using cold potentials in the earth near the surface of the earth in low depths, as well as by using cold potentials, which are created by expansion processes from other gases as natural gas and air and are delivering wanted low temperatures.

The process will be used as a combination of a gas power station with natural heat and cold potentials. Big amounts of carbon dioxide will be stored in the buffers of carbon dioxide and used immediately in a discontinuous working and by expansion for the production of electrical power too without significant start-up and shut-down periods.

The start of development of the storage for carbon dioxide is made by winning carbon dioxide from the exhaust air by condensation of water for removing (drying) first and after then the carbon dioxide by compression over 5 MPa, cooling to a temperature of 281 to 283 K and condensing. Cooling is made by the earth temperature in 3 to 30 meters depth. The liquid carbon dioxide is collected and given through the buffer storage into the underground storage. A part of this can be used for substituting losses of the circuit in the liquid range. For commissioning the plant the carbon dioxide circuit must be filled with carbon dioxide from other sources. In the winter time can be used the surrounding air temperature for cooling when the temperature is below 278 K. Then can be is used a lower pressure for the liquefaction, depending from the real air temperature.

EXAMPLES OF APPLICATION

Further advantages are given by the description of examples of application of the invention as well as the connected picture and table.

The fundamental principle of the application of the process and the device for using of the waste energy of an energy generation plant by using the earth as cooling source for condensation of the working fluid carbon dioxide is shown in the picture. Three different heat potentials at 363 K, 373 K und 623 K are used exemplary as heat sources at the working pressure at 15 MPa in the examples I to III. As expansion machine (2) is used an expansion turbine. The earth heat potential in the depths of 8 to 30 meters is used as cold source (4) for the condensation of the working fluid carbon dioxide which was expanded to 4.5 MPa. A pressure chamber (6) will used as a temporary storage. The pipes for the carbon dioxide circuit are the lines 7 to 11 according to the picture. The calculation of the examples was made with the program EBSILON Professional. In example IV of the second part of the table is calculated the situation of example II (heat source at 373 K) with a lower air temperature of 271 K, which is characteristic for the winter, as cold source (4) and their using by air coolers. For this it is given a lower turbine output pressure of 3.7 MPa. By using of the broader ranges of temperature and pressure the efficiency of energy generation plant is 1.3% better than in example II. This result is important especially of regions with low temperatures as well as for using of geothermic energy and power stations.

The low temperatures are causing a relatively low efficiency but in all cases the efficiency of this process was more than 2% higher than in comparable processes. In a co-generation the heat source (1) of the process was the waste energy. In the examples I to IV the waste energy is used at the given temperature levels 363 K, 373 K, and 623 K and should be used energetically. The fluid carbon dioxide is streaming from the underground buffer (6) with a pressure of 15 MPa and temperatures of 293.5 K (examples I to III) and 284.5 K (example IV) through the heat exchanger (1), heated at the given temperatures to 363 K (example I), 373 K (example II and IV), and 623 K (example III), streaming through a control valve to an expansion turbine (2), expanding labor-producing from 15 MPa to 4.5 MPa (examples I to III) or rather 3.7 MPa (example IV) and driving on this way a with the turbine connected generator (3). The carbon dioxide was expanded into an underground pipe network as the cold source (4) to a temperature of 281 K in the examples I to III. The carbon dioxide is liquefied at this temperature because the long retention period of the carbon dioxide in the pipe network. An air cooler was used as the cold source (4) at 271 K in the example IV and the carbon dioxide was expanded to a pressure of 3.7 MPa. The liquid carbon dioxide is going through a heat isolated pipe (9) to the liquid pump (5), compressed to a pressure of 15 MPa, and stored in the buffer (6). The work for the compression of the liquid carbon dioxide is lower than 30% of the produced energy. The netto-efficiency of the process is given in the table. The efficiency is increased at higher heat potentials and lower cold sources are usable, e.g. by using of the expansion cold of natural gas and can reach at 373 K efficiencies of nearly 25%.

TABLE Fluid Temperature Pressure Power KW Elt Elt Netto flow Unit K MPa Therm. Electr. Brutto Netto Efficiency Example 7 363 15 I 2, 3 328 8 283 4.5 4 −1809 9 283 4.5 5 −119 10, 11 293.5 15 1 2018 328 209 10.4% 7 373 15 II 2, 3 450 8 283 4.5 4 −1902 9 283 4.5 5 −119 10, 11 293.5 15 1 2243 450 331 14.4% 7 623 15 III 2, 3 1230 8 493 4.5 4 −4486 9 283 4.5 5 −119 10, 11 293.5 15 1 5598 1230 1112 19.9% 7 373 15 IV 2, 3 514 air 8 275.5 3,7 cooling 4 −2049 9 271.5 3.7 5 −121 10, 11 284.5 15 1 2442 514 393 16.1% 

1. A process for using of low temperature heat for the production of electric power, the process comprising: using supercritical carbon dioxide as working fluid, in which high compressed supercritical carbon dioxide as heat transfer fluid takes off low temperature heat of a heat source and then is expanded in a labor-producing expansion machine, which is connected with a generator, is cooled in this process and liquefied by means of a cold source, then increased of pressure by a pump for liquids to the working pressure and stored in a high pressure interim storage.
 2. A process as claimed in claim 1, wherein the labor-producing expansion is made into the range of vapor-liquid-equilibrium with a partial condensation of carbon dioxide and the vapor-liquid-mixture is liquefied totally by means of a cold source, then increased of pressure by a pump for liquids to the working pressure and stored in a high pressure interim storage.
 3. A process as claimed in claim 1, wherein as interim storages are used salt caverns in big depth.
 4. A process as claimed in claim 1, wherein the waste heat of a power station is used as heat source.
 5. A process as claimed in claim 1, wherein the waste heat of motors is used as heat source.
 6. A process as claimed in claim 1, wherein the waste heat of machines and plants is used as heat source.
 7. A process as claimed in claim 1, wherein geothermal energy is used as heat source.
 8. A process as claimed in claim 1, wherein solar energy is used as heat source.
 9. A process as claimed in claim 1, wherein the geothermal potential in depth of 5 to 30 meters is used as cold source for the liquefaction of the carbon dioxide at last partially.
 10. A process as claimed in claim 1, wherein the ambient air or substances which are tempered by the ambient air are used as cold source for the liquefaction of the carbon dioxide at last partially.
 11. A process as claimed in claim 1, wherein deep water of seas, rivers or oceans or substances which are tempered by deep water are used as cold source for the liquefaction of the carbon dioxide at last partially.
 12. A process as claimed in claim 1, wherein the cold energy of the expansion of compressed air or natural gas or substances which are tempered by the cold energy of the expansion are used as cold source for the liquefaction of the carbon dioxide at last partially.
 13. A process as claimed in claim 1, wherein the labor-producing two-stage expansion is characterized by an expansion into the two-phase region in the first step by separating of the gas part and using of a liquid part of this stream for cooling of the whole stream for the expansion, in this process will be the stream heated and then in a second step expanded to lower pressures, then used for the cooling and liquefying of the first gas stream and after them compressed and assembled with the liquefied first gas part and in the liquid form compressed to the working pressure.
 14. A process as claimed in claim 1, wherein the salt caverns in the process are used both storages for carbon dioxide in the supercritical high pressure state and geothermic heat sources and heat exchanger, through which the potential of the carbon dioxide emission is lowered additional
 15. A process as claimed in claim 1, wherein the liquefaction takes place in the earth near the surface of the earth and the depth storage in more than 400 meters because the high pressure of the carbon dioxide of 10 MPa at least by safety reasons and the high static pressure the costs of the compression reduces.
 16. A process as claimed in claim 1, wherein the process is operating in joint with a peak load power station of natural gas basis and operating discontinuously, through which the excess energy is used to load natural gas, compressed air and carbon dioxide in underground buffers under high pressure and, when needed, to take off air and natural gas discontinuously and the storage for carbon dioxide to use both as buffer storage and as geothermal heat source for the working fluid.
 17. A device realizing the process as claimed in claim 1, wherein is used at least a given heat source, a heat exchanger with liquefying function, a medium for the heat transfer, an expansion machine, a generator which is connected with the expansion machine, a pump for the compression of the liquid carbon dioxide, a buffer for the storage of the liquid working fluid, control devices and valves. 